Embodiments in accordance with the present invention relate to magnetic disk drives such as a hard disk.
Hard disks adopt a method for reliably writing data to a desired area on a rotating magnetic disk. This method is called a sector servo system. According to the sector servo system, servo information is written at a constant angular interval to each track on a surface of the magnetic disk. A circuit unit in a hard disk drive reads and detects this servo information, and then reads or writes data with reference to a position of the servo information.
Here, if the rotation center of a magnetic disk deviates (if the rotation center is shifted), intervals of detecting servo information fluctuate, which may exert an influence on the positioning accuracy at the time of reading or writing data.
For this reason, heretofore, fluctuations in servo interval time are detected and the servo interval time is subjected to feedforward correction in expectation of the fluctuations (for example, as disclosed in Japanese Patent Application Laid-Open No. 2004-139677).
In recent years, not only computers but also small-size information equipment including mobile terminals and car navigation systems have made use of a hard disk as means for providing large storage capacity. Under such circumstances, when a hard disk drive is implemented in a mobile terminal or the like, the hard disk drive may suffer a shock if the mobile terminal drops. Likewise, when a hard disk drive operates while a vehicle is running, the hard disk drive may be subject to consecutive disturbance.
In particular, if the mobile terminal is fixed to one's arm with an armband, or if the mobile terminal is moved with the mobile terminal held by hand, a disturbance may be applied to the mobile terminal which can be described mathematically by a large arc or a parabolic equation. The disturbance as described above acts in the circumferential direction (in the rotational direction, i.e., in the angular speed direction) of the magnetic disk, which causes fluctuations in rotation of the magnetic disk. This may exert an influence on the positioning accuracy of write and read positions.
However, with conventional hard disk drives, the disturbance in the circumferential direction of a magnetic disk is not taken into consideration. Therefore, when such disturbance causes servo intervals to fluctuate, correction processing for the disk shift may be inaccurately performed. As a result, it may become more difficult to control write and read positions.